But in many cases the internet is provided by the cable company.
You're not following me. Voice or telephone from a cable company is not carried by the internet. From the cable modem it goes back to their head end or office using their coax and fiber then out to the local telco central office using the usual telco methods. This is why service is good.
Vonage utilizes the internet to go from the cable company to the Vonage switch, wherever in the country that might be. This is why service quality varies greatly.
With either carrier you are going to have to contend with loss of power causing your service to go down. Cable companies have backup batteries in all their plant power supplies as does the telephone companies. Both have generators to keep the central office and cable head ends up and running. You can always provide a UPS for your modem. I know that with an extended outage you will still go down but you need to keep that in mind when considering the service vs price.
-Hal
